Project 3 is designed to probe the neurobiological underpinnings of age-related cognitive decline in female rhesus monkeys as well as the capacity of various unopposed estrogen treatments (ET) and/or combined hormone treatments (HT) to sustain a neuronal and synaptic phenotype that preserves cognitive function. We are particulariy interested in the synaptic characteristics of excitatory circuits in dorsolateral prefrontal cortex (dIPFC) and hippocampus that are predictive of cognitive performance and/or differentially affected by age and ET or HTs with high clinical relevance. We will also reveal the synaptic repercussions of altering the time of onset and duration of treatment following ovariectomy (OVX) and how these synaptic indices relate to the cognitive effects of delaying or terminated treatment. These studies will impact our knowledge of synaptic and molecular links to cognitive performance and age-related cognitive decline as well as the effects of multiple variations in the design and timing of ET/HT on key synaptic indices, providing new potential therapeutic targets and strategies to sustain optimal synaptic and cognitive health. Synaptic indices are obtained through quantitative neuronal reconstruction, confocal analyses, and postembedding immunogold electron microscopy (EM), and correlated with cognitive performance through regression analyses. Specific Aim 1 will use archival tissue from young and aged ovariectomized (OVX) female rhesus monkeys that received vehicle or cyclical ET to determine the key molecular attributes of the axospinous synapses that are vulnerable to aging and responsive to ET, and which synaptic characteristics predict sustained cognitive performance seen in aged monkeys with this ET regimen. Key molecular targets will be NMDA and AMPA glutamate receptors, synaptic estrogen receptors (e.g., ERa, ERp, and GPR30), and signaling molecules linked to spinogenesis that emerge from Project 1 and have been analyzed in the rat model (e.g., PELP1). Specific Aim 2 will use similar approaches to define the spine and synaptic effects of HT regimens that employ cyclical or chronic E with and without cyclical or chronic progesterone (P) to determine how these regimens compare to cyclical unopposed E. Specific Aim 3 will determine whether or not there is a window of opportunity for beneficial synaptic effects of ET/HT after cessation of ovarian function, and how long the beneficial effects endure without continued treatment. In all cases, we will study the brains of animals at the conclusion of cognitive testing. These results will provide critically important information for the design of HTs that provide maximal neurological benefits for post-menopausal women. RELEVANCE (See instructions): This project is designed to reveal the interactive effects of age and estrogen on the brain circuits that mediate cognition. The proposed experiments will provide a conceptual and neurobiological framework for the design of more effective hormone treatments for menopausal women than those that are currently in use